1. Field of the Invention
The invention relates to methods for measuring a filling material level in vessels and for detecting filling material presence in vessels.
2. Description of the Related Art
The need to know about the presence of a filling matter in a vessel, as well as the quantitative information about the space that the filling matter occupies in the vessel, is evident in many industrial and domestic applications. Accordingly, a variety of methods and devices are described in the scientific and technical literature. Methods for filling material level measurement could be grouped by the following five considerations:                1. Invasive vs. Non-invasive        2. Dependence of the method upon the vessel's material        3. Dependence of the method upon the filling material        4. Invariance to environmental disturbances        5. Safety        
Of these, the primary consideration is invasive versus non-invasive because this consideration is the most serious constraint for the measuring method and the measuring apparatus implementing the method. Therefore, the prior art methods will be discussed from the position of invasiveness first of all. Invasive methods require the presence of a measuring device's element inside the vessel; non-invasive methods are limited to those that do not require a measuring device's element inside the vessel. The number of invasive methods is noticeably larger then the number of non-invasive methods. The former methods could utilize the principals of Time Domain Reflectometry of very short electrical pulses (TDR) that “ . . . are propagated along a transmission line or guide wire that is partially immersed in the material being measured. . . . Reflected pulses are produced at the material interface due to the change in dielectric constant. . . . ” The time difference between the launched and reflected pulses is used to determine the material level, as disclosed in U.S. Pat. Nos. 5,610,611, 6,452,467 and 6,481,276. This approach is usable in continuous and set-point level measurement applications.
Another known approach to the material presence detection and the filling material level measurement is based on monitoring the dynamic properties of a mechanical system comprised of an oscillatory structure directly contacting the filling material as disclosed in U.S. Pat. Nos. 5,631,633, 4,896,536, 6,105,425 and 5,862,431. Some solutions suggest using a mechanical arm with one end fixed at a predetermined level and the opposite end attached to a switch, to provide for a level relay control or set point level measurement of the filling material. U.S. Pat. No. 6,111,211 issued to Dziedzic et al, serves as an example of the above-described invasive method for the set point level measurement. Among the methods that exploit monitoring the dynamic properties of a mechanical system, U.S. Pat. No. 4,954,997 issued to Dieulesaint, et al. represents a set-point liquid level measurement solution that monitors changes in the parameters of the Lamb elastic waves in the detection plate. These waves are generated by a transmitter and are received by a receiver of the measuring system. The plate is installed at a predetermined level inside of a tank. The parameters of the Lamb wave change dramatically at the moment the detection plate contacts the filling liquid, thereby allowing the set-point level measurement. Scientific Technologies, Inc., manufactures another vibrating level sensor, VBS series. The sensor is described in the company website www.stiautomationproducts.com: “The VBS series is designed specifically for solid level detection in very small hoppers less than 3 ft (1 m) tall. The VBS is a compact diaphragm vibration switch for use with dry solids at atmospheric pressures.” “The sensitivity may vary depending on the apparent specific gravity and fluidity of the powder,” thus the detection sensitivity depends on the buried portion of the diaphragm in the vertical direction.
A large group of methods is based on the capacitive properties of the filling material. According to these methods, at least one member of the measuring capacitor is located within the container. The electrical capacitance of the measuring capacitor varies depending on the amount of filling material and could be calculated to correspond to the measured level. U.S. Pat. Nos. 5,207,098 and 4,574,328 illustrate such invasive capacitive methods.
Although limited by the constraints of specific applications, every ultrasound, electromagnetic, and laser method for distance measurement and their combinations are usable for measuring the filling material level in vessels. Some of these methods are disclosed in U.S. Pat. Nos. 5,877,997, 5,793,704, 6,122,602, 5,822,275, 5,699,151, 6,128,982 and 5,892,576 and illustrate the wave-train-based invasive approaches to the vessel's filling material level measurement.
With regard to non-invasive methods for the level measurement, the following popular approaches are known:                Radioactive        Capacitive        Ultrasound        Gravitational        
Radioactive methods are based on the fact that radioactive energy attenuates after passing through a vessel's walls and through filling material. Obviously, radioactive systems are dependent on the vessel's material and the filling material. These systems are not capable of continuous level measurement and these systems require special design and operational efforts to maintain a sufficient degree of safety. The example of a radioactive system usable for a set point level measurement is Radiometric Measuring System DG57 manufactured by Endress+Hauser.
Gravitational systems require the exact knowledge of the empty vessel's weight and its dimensions including the internal dimensions. Gravitational systems are limited in their applicability due to problems with installation of the weight-measuring equipment and calculation of the actual level of filling material, which varies depending on the vessel's internal topology, mechanical properties of the filling material and environmental conditions, e.g., material viscosity or temperature. Vishay Nobel of Sweden manufactures one such gravitational system.
Non-invasive capacitive methods for the material level measurement in vessels are subject to a very strong limitation. In order to obtain satisfactory measurement resolution, the distance between the conductive elements of a sensing capacitor must be substantially smaller than their area. In fact, the capacitance of two parallel rectangular plates taken into the first approximation analysis is described by a well-known formula:
                    C        =                              k            ⁢                                                  ⁢                          ɛ              0                        ⁢            A                    d                                    (        1        )            wherein, C denotes the capacitance; εo—electric constant; k—relative permittivity; A—area of a flat rectangular conductive element; and d—distance between the conductive elements. Note that all mathematical notations in this patent application are standard (refer to www.mathworld.wolfram.com). Given that d=0.01 m, A=1.00 m2, k=2.5 (typical value for dielectric materials), C=2.5·8.85·10−12·1.00/0.01=22.125·10−4 μF. A 10% change in the area of the conductive plates results in a 221.25 pF change in the capacitance. This value is comparable with the capacitance of wiring for a printed circuitboard. Thus, the non-invasive capacitive method for level measurement is only practically applicable to very small vessels with dielectric filling material. For the same reason, in applications with electro-conductive filling material, the non-invasive capacitive methods are only feasible for vessels with relatively thin non-conductive walls. Therefore, non-invasive capacitive methods for the filling material level measurement in vessels have a narrow field of application. U.S. Pat. Nos. 6,448,782 and 6,472,887 offer a detailed description of devices utilizing the non-invasive capacitive method for filling material level measurement in vessels; the former is for electro-conductive filling material, and the later is for dielectric filling material.
Ultrasound non-invasive methods for level measurement require the attachment of one or more transducers to the external wall of a vessel for transmitting the acoustic energy toward the boundary surface separating the filling material from the remaining space inside of the vessel. The receiver of the measuring system gets the reflected ultrasound wave train and sends it to the device's echo processing electronics. Thus, with the exception of the external attachment of the transducers, these methods bear all the distinctions of well-known invasive ultrasound methods for the distance/level measurement. However, the ultrasound non-invasive method is advantageous because of its non-invasiveness. At the same time, the ultrasound non-invasive approach to the filling material level measurement is limited by the homogeneity of the filling material. Typically, measuring systems of this method are used for homogeneous liquid filling materials. It is not applicable to loose materials or liquids with inclusions. In addition, this method is not applicable to relatively small-sized containers due to problems with acoustic pulse relaxation, reverberation and the size of transducers. Plus, the method is temperature-dependent, thereby requiring temperature compensation during measurement. If used for the set-point level measurement or material presence detection, the method is prone to creating false alarms due to the effect of some volume of a viscous filling material adhering to the internal surface of the container. Finally, the ultrasound-based non-invasive technologies require special treatment of the container's surface in order to create a conduit for ultrasound waves emitted by a transducer into the container. An example of such technology is VesselCheck ST and SpotCheck of Cannongate Technology, UK. The VesselCheck marketing material published on www.cannongatetechnology.co.uk says: “With VesselCheck ST, there's no need to make holes in the vessel. The unique VesselCheck ST provides continuous measurement with no process connections, meaning nodown-time during installation. Two small ultrasonic transducers are clamped to the outside walls of the vessel. One is mounted on the bottom of the vessel and the other on the side, to compensate for variations in temperature and density.” “SpotCheck uses an ultrasonic “footprint” to determine the presence or absence of liquid inside a tank or pipe. . . . In order to insure that SpotCheck will operate satisfactorily, the surface of the tank or pipe must be prepared correctly.” For instance, deseaming and gelling the wall's surface is required in the area in which the transducer is mounted.
A similar approach has been announced by HiTECH Technologies, Inc., USA. This company markets continuous and set point devices based on their Penetrating Pulse Technology (PPT) [Online article: “Penetrating Pulse Technology,” at www.hightechtech.com], which resembles the method developed by Cannongate Technology, Ltd., UK. The distinctive feature of PPT is generating a single short ultrasound impulse penetrating the vessel's wall toward the filling material. The HiTECH Technologies-developed SONOMETER for the continuous level measurement and SONOCONTROL for the set point level measurement are based on PPT. The company provides a comprehensive description of their method on the website www.hitechtech.com. An analysis of PPT methods shows that the technology is ultrasound and uses either the Pulse Transit Time paradigm or the monitoring of the duration of ultrasound waves' relaxation to indicate the material presence in the plane in which the special acoustical transducer is installed on the outside wall of the vessel. The PPT are subject to all of the above-described limitations of the ultrasound methods of the level measurement.
An analysis of the related art shows that all known invasive or non-invasive level measurement techniques are limited by the factors of vessel's material, filling material and environment. See also: Burdik V. Analysis of Sonar Systems. L., 1988; Skoochik E. Fundamentals of Acoustics. —., 1976; and Krasilnikov V._., Krylov V. V. Introduction to Physical Acoustics. —., 1984.
The object of the present invention is to develop a method for the non-invasive measurement of the filling material level in the vessel free of the underlined limitations.